Date of Award:

5-2024

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Committee Chair(s)

Brady R. Cox

Committee

Brady R. Cox

Committee

James A. Bay

Committee

Mohsen Zaker Esteghamati

Abstract

As earthquake waves exit rock and travel up through softer subsurface strata on their way to the surface, the intensity of ground shaking is significantly influenced by the subsurface soil layering and often increases in areas with deeper, softer soil deposits. The soft soils present in Utah’s valleys, deposited during the era of Lake Bonneville, tend to amplify the energy and intensity of ground motion felt by people and structures alike. Thus, the first step to designing earthquake resilient infrastructure in Utah is understanding how the subsurface soil layers amplify and/or attenuate ground motions induced by an earthquake. Methods used to predict surface shaking at a specific site are known as ground response analyses (GRA).

The I-15 Downhole Array (I15DA) is located near the south intersection of I-15 and I-80 in South Salt Lake, Utah. The array is made up of five accelerometers installed in boreholes at different depths (0 m, 7.6 m, 18.3 m, 48.4 m, and 119.8 m) to record ground motions as they travel up towards the ground surface. The array recorded some of the aftershocks following the magnitude 5.7 2020 Magna, Utah Earthquake. By comparing the characteristics of the recorded ground motions between a given reference accelerometer at depth and an accelerometer at the surface, one can quantify how the ground motions are altered and amplified as they travel through the near surface strata.

Unfortunately, there is not a downhole array at every location where there is a need for understanding seismic ground response in Utah. Instead, engineers perform GRAs to estimate the theoretical ground response through analytical solutions and numerical modeling. However, without the ability to compare predictions with actual recordings of ground motions at different depths, there is uncertainty about how well the GRAs predict the site response. This study seeks to evaluate the effectiveness of many common approaches for incorporating uncertainty and spatial variability in shear-wave velocity (Vs) by validating each studied method against the recorded ground motions at the I15DA site. The I15DA site is representative of many sites around Utah’s valleys. Understanding which GRA methods are appropriate for the I15DA will help identify appropriate site response methods for other sites and will help to design seismically resilient infrastructure in the state of Utah.

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